I did the same thing - messed around with MailChimp & then realized free version doesn't have an autoresponder.
My personal pick is VerticalResponse because they are packed with features and are one of the few email services that offer pay as you go. If you have a smaller list and do not email frequently it can be MUCH cheaper (like around 1-2 cents per email sent).
Free trials are always the way to go to ck them out - especially with email services. I've done trials of pretty much all of them at some point!
There's a list of some of the top ones and basic features on my site here: